Vector: make operator-() a const function
UnitTest: add tests for equality/inequality
UnitTest: merge .cpp file into .hpp

Function templates get only defined upon instantiation. That way we were
lucky that the linker didn't barf from multiple definitions of the
assert* functions. This is NOT true for the non-template functions
assertTrue/assertFalse previously introduced. For a workaround, we merge
everything into a single header file to be imported.

In the long run, we need to heavily refactor and restructure the
unittest code.
tests: test for copy constructor post-condition

I had the fear that the default copy constructors for the Matrix class
were wrong and that only the pointer to the C-like array gets copied. It
seems not the case but I actually don't quite understand why.

In the long term, I think it is better to replace the C-array by a STL
vector/array. We should avoid raw-pointers at all costs, they say.
tests: rotation of vectors shouldn't change norm
Matrix: remove trailing newline